Cathodic sputtering is widely used for the deposition of thin layers of material onto desired substrates. Basically, this process requires a gas ion bombardment of a target having a face formed of a desired material that is to be deposited as a thin film or layer on a substrate. Ion bombardment of the target not only causes atoms or molecules of the target materials to be sputtered, but imparts considerable thermal energy to the target. This heat is dissipated beneath or around a backing plate that is positioned in a heat exchange relationship with the target. The target forms a part of a cathode assembly that, together with an anode, is placed in an evacuated chamber filled with an inert gas, preferably argon. A high voltage electrical field is applied across the cathode and the anode. The inert gas is ionized by collision with electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, these ions dislodge the target material. The dislodged target material traverses the evacuated enclosure and deposits as a thin film on the desired substrate, which is normally located close to the anode.
In addition to the use of an electrical field, increasing sputtering rates have been achieved by the concurrent use of an arch-shaped magnetic field that is superimposed over the electrical field and formed in a closed loop configuration over the surface of the target. These methods are known as magnetron sputtering methods. The arch-shaped magnetic field traps electrons in an annular region adjacent to the target surface, thereby increasing the number of electron-gas atom collisions in the area to produce an increase in the number of positive gas ions in the regions that strike the target to dislodge the target material. Accordingly, the target material becomes eroded in a generally annular section of the target face, known as the target raceway.
In a conventional target cathode assembly, the target is attached at a single bonding surface to a nonmagnetic backing plate to form a parallel interface in the assembly. The backing plate is used to provide a means for holding the target assembly in the sputtering chamber and to provide structural stability to the target assembly. Also, the backing plate is normally water-cooled to carry away the heat generated by the ion bombardment of the target. Magnets are typically arranged beneath the backing plate in well-defined positions to form the above-noted magnetic field in the form of a loop or tunnel extending around the exposed face of the target.
To achieve good thermal and electrical contact between the target and the backing plate, these members are commonly attached to each other by use of soldering, brazing, diffusion bonding, mechanical fastening or epoxy bonding.
Smooth surface diffusion bonding is an applicable method of bonding, but has only limited use in the bonding of sputtering target components. The bond is produced by pressing the material surfaces into intimate contact while applying heat, to induce metallurgical joining and diffusion to a varying extent across the bond interface. Bonding aids, metal combinations which are more readily joined, are sometimes applied to one or both of the surfaces to be bonded. Such coatings may be applied by electroplating, electroless plating, sputtering, vapor deposition or other usable techniques for depositing an adherent metallic film. It is also possible to incorporate a metallic foil between bonding members that has the ability to be more easily bonded to either of the materials to be joined. The surfaces to be joined are prepared by chemical or other means to remove oxides or their chemical films which interfere with bonding.
Smooth surface diffusion bonding requires extreme care in preparation and in maintaining surface cleanliness prior to and during the bonding operation to ensure reliable bond qualities. Because the diffusion bond interfaces are planar, they are subject to stressing in simple shear which commonly leads to peeling away at the ends of the bond area. The formation of brittle intermetallics at the bond interface, which increase in thickness with the associated long times of heat exposure, add to the potential of bond shear failure. An additional technique for bonding, as described in U.S. Pat. No. 5,230,459 includes the pre-bonding step of providing machined grooves in the surface of one of the components to be solid-state bonded. This feature causes disruption of the bond surface of the associated component during heated pressure application. The material having the greater strength of hardness will normally be provided with the grooves such that, during bonding, it will penetrate into the softer member with the softer metal substantially filling the grooves.
Groove bonding is applicable to bonding many dissimilar materials, but is limited to materials that have dissimilar melting temperatures because the process must occur near the melting temperature of the lower melting point alloy. This precludes the use of this technique for similar metals. It is also possible that the saw tooth nature of the grooves may act as a stress concentrator and promote premature cracking in the alloys near the bonds. Furthermore, machining of the grooves is a time consuming operation.
In U.S. Pat. No. 5,836,506, hereby incorporated by reference in its entirety, a method is disclosed for performing a surface roughening treatment to the bonding surface of the sputter target and/or backing plate, followed by solid state bonding. This roughening surface treatment provides 100% surface bonding compared to only 99% surface bonding in the absence of the surface treatment. The treatment further provides a bond with over twice the tensile strength of a bond formed from the non-treated smooth surfaces.
In all of the above diffusion bonding processes, elevated temperatures of varying degree are applied to form the bond between the target and the backing plate. Thus, in each of these processes, changes in the microstructures of the target and backing plate materials are likely to occur because prolonged exposure of metals to elevated temperatures causes grain growth. Great strides have been made in this art to process sputter target blanks to achieve certain microstructures that are linked to increased sputtering efficiency and improved thin film quality. After a desired microstructure is obtained in the sputter target, the microstructures could be in jeopardy of being altered by elevated temperature bonding methods for attaching the target to the backing plate.
Additionally, although diffusion bonding has been proven successful, extra large target/backing plate assemblies require large scale diffusion bonding presses and this poses a significant capital expenditure.
Prior art attempts to solve the problem of distorting the microstructure of the sputter target and maintaining a consistent and uniform bond, requires the placing of wire gauges between the sputter target and backing plate. This method is labor intensive, costly and the wire spacer gauges tend to move during the bonding process. Additionally, when the wire gauges are used on the outer edges of the sputter target assembly, bowing of thin or large targets can occur due to an inconsistent thickness of sputter target material. Thickness uniformity of sputter targets is particularly important for ferromagnetic materials in order to achieve good thickness and sheet resistance uniformity of sputtered films.
It is an object of the invention to provide a method of forming a bonded sputter target/backing plate assembly that has a uniform thickness bond interface and uniform flatness of the target sputtering surface.
Another object of the invention is to provide a bonded sputter target/backing plate assembly that does not compromise the microstructural characteristics of the sputter target.
Another object of the invention is to provide a bonded sputter target/backing plate assembly having at least two spaced-apart peripheral flange segments disposed on the bonding surface of the backing plate and at least two peripheral notch segments to accommodate the at least two peripheral flange segments on the backing plates.